Remove redundant memory allocation in Node::insert

[MegaCorn]

If the inserted node is not a fragment, unnecessay vector memory allocation will cause 20% performance degradation

Testing: By comparing flame graph Fixes: https://github.com/servo/servo/issues/37223

[github-actions[bot]]

🔨 Triggering try run (#15562670467) for OpenHarmony

[github-actions[bot]]

Bencher Report

Branch	main
Testbed	HUAWEI Mate 60 Pro

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Benchmark	Latency	milliseconds (ms)	Memory	Bytes
E2E/https://www.google.com/JS/gc-heap/admin			📈 view plot ⚠️ NO THRESHOLD	23,840.00
E2E/https://www.google.com/JS/gc-heap/decommitted			📈 view plot ⚠️ NO THRESHOLD	573,440.00
E2E/https://www.google.com/JS/gc-heap/unused			📈 view plot ⚠️ NO THRESHOLD	49,464.00
E2E/https://www.google.com/JS/gc-heap/used			📈 view plot ⚠️ NO THRESHOLD	401,832.00
E2E/https://www.google.com/JS/malloc-heap			📈 view plot ⚠️ NO THRESHOLD	2,862,451.00
E2E/https://www.google.com/JS/non-heap			📈 view plot ⚠️ NO THRESHOLD	262,144.00
E2E/https://www.google.com/LayoutThread/box-tree			📈 view plot ⚠️ NO THRESHOLD	101,488.00
E2E/https://www.google.com/LayoutThread/display-list			📈 view plot ⚠️ NO THRESHOLD	0.00
E2E/https://www.google.com/LayoutThread/font-context			📈 view plot ⚠️ NO THRESHOLD	4,040.00
E2E/https://www.google.com/LayoutThread/fragment-tree			📈 view plot ⚠️ NO THRESHOLD	112.00
E2E/https://www.google.com/LayoutThread/stylist			📈 view plot ⚠️ NO THRESHOLD	5,264.00
E2E/https://www.google.com/Load	📈 view plot ⚠️ NO THRESHOLD	379.73 ms
E2E/https://www.google.com/Resident			📈 view plot ⚠️ NO THRESHOLD	183,098,572.00
E2E/https://www.google.com/image-cache			📈 view plot ⚠️ NO THRESHOLD	35,328.00
E2E/https://www.servo.org/Load	📈 view plot ⚠️ NO THRESHOLD	1,025.47 ms
E2E/https://www.servo.org/Resident			📈 view plot ⚠️ NO THRESHOLD	261,003,673.00

🐰 View full continuous benchmarking report in Bencher

[github-actions[bot]]

✨ Try run (#15562670467) succeeded.

[servo-wpt-sync]

🛠 These changes could not be applied onto the latest upstream WPT. Servo's copy of the Web Platform Tests may be out of sync.